1. Field of the Invention
This invention relates to assay for an analyte, and more particularly relates to membrane flow-through assay and a device useful therein.
2. Background of the Invention
Assay systems which are both rapid and sensitive have been developed to determine the concentration of a substance, generally referred to as the analyte, present in low concentration in a fluid sample. Immunoassays depend on the binding of an antigen or hapten to a specific antibody and have been particularly useful because they give high levels of specificity and sensitivity. These assays employ one of the above reagents in labeled form, the labeled reagent being referred to as the tracer.
Enzymes have often been used as labels in immunoassay. In conventional enzyme immunoassay (EIA), an enzyme is covalently conjugated with one component of a specifically binding antigen antibody pair, and the resulting enzyme conjugate is reacted with a substrate to produce a signal which is detected and measured. The signal may be a color change, detected with the naked eye or by a spectrophotometric technique, or may be conversion of the substrate to a product detected by fluorescence.
A convenient format for EIA is solid phase immunoassay in which one of the assay reagents is immobilized on a solid support. The solid support may be in the form of a dipstick, the inside wall of a test tube or cuvette or the well of a microtiter plate. A particularly useful solid support is a microporous membrane.
Membrane assay is often referred to as flow through assay since the assay steps are performed sequentially with the fluid phase of each step passing through the membrane before the next step is initiated. An assay device in which fluid flow is gravity controlled is disclosed in U.S. Pat. No. 4,111,754 to Park. Flow-through assay devices in which flow is enhanced by capillary action induced by an absorbent pad in contact with the membrane are disclosed by U.S. Pat. No. 3,888,629 to Bagshaw and U.S. Pat. No. 4,632,901 to Valkirs et al. A dipstick assay device using flow-through is disclosed by Tom et al. in U.S. Pat. No. 4,366,241. In the Bagshaw, Valkirs et al. and Tom et al. devices, fluid flow through the membrane cannot be controlled by the technician. Cole et al., in U.S. Pat. No 4,246,339 discloses a device in which capillary action is controlled by biasing the absorbent pad into and out of contact with the membrane. Devices in which flow is initiated by vacuum are described in U.S. Pat. No. 4,277,560 to Gray and U.S. Pat. No. 4,812,293 to McLaurin et al.
While the devices disclosed in the above patents using mechanical means to promote flow have advanced the art of flow through assay, there remains a need for a device in which flow can be controlled merely by passing a flow controlling reagent through the membrane. It is toward fulfillment of this need that the present invention is directed.